The present invention relates to equipment of the type used in surveying and construction and, more particularly, to a detector device for such applications which has an improved photodetector arrangement for detecting the position of reference light. The light may typically be a rotating laser beam that defines a reference plane of light or a stationary reference plane or cone of laser light. The reference plane may be horizontal or inclined, as dictated by the application.
Laser systems have commonly been employed in surveying and construction in which a laser beam is rotated in either a horizontal or a graded plane. U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando, illustrates a laser transmitter which provides such a rotating reference beam. The rotating beam defines a plane, and various measurements can be made using the plane as a reference. For example, the elevation of a point remote from the laser transmitter may be measured using a rod on which a laser receiver is mounted. The bottom of the rod rests on the ground at the point where the measurement is to be made, and the operator moves the receiver along the rod to a position where it intercepts the laser beam, as indicated by a display on the receiver. One such laser receiver is shown in U.S. Pat. No. 4,240,208, issued Jun. 30, 1987, to Pehrson.
A similar surveying system is shown in U.S. Pat. No. 4,732,471, issued Mar. 22, 1988, to Cain et al. In the Cain et al. system, a laser transmitter produces an alignment field by projecting laser energy in a non-planar, stationary reference cone. The Cain et al. patent discloses a small, hand-held laser receiver which includes a display and a photodetector module. The receiver may be moved about a worksite to detect the height of the reference cone at various locations.
A number of laser detector or receiver devices have included circuitry that provides an indication of the position of the detected reference light with respect to the device as a ratio between several output signals. U.S. Pat. No. 4,907,874, issued Mar. 13, 1990, to Ake discloses such a detection and display device that includes first and second interdigitated photodetector elements. The photodetector elements are each made up of a plurality of sections which are arranged in a generally vertically oriented row. The heights of the sections of one of the elements increase from the bottom of the row to the top of the row, while the heights of the sections of the other element decrease from the bottom of the row to the top of the row. An enhanced detection and display device is shown in U.S. Pat. No. 4,976,538, issued Dec. 11, 1990, to Ake. The device has first and second interdigitated photoconductor elements that are each made up of a plurality of sections arranged in a generally vertically oriented row. In the same manner as shown in the '874 patent, the heights of the sections of one of the elements increase from the bottom of the row to the top of the row while the heights of the sections of the other element increase from the top of the row to the bottom of the row. Unlike the sections of the device of '874 patent, however, the sections of the elements in the device of the '538 patent are inclined with respect to the row. As a consequence, a beam sweeps across the sections in a manner providing a continuously variable output ratio.
While these detection devices provide accurate detection of laser light in a reference plane, they require the use of specially constructed photodetector elements having specific design geometries. Such photodetector elements are expensive, and add significantly to the overall cost of the detection devices. These devices do have the advantage, however, of reducing the amount and complexity of the necessary signal processing circuitry, since the position of the detected light is supplied to the circuitry as simply the ratio of a pair of detection signals. General purpose, photodetector elements in the form of PIN photodiode arrays, on the other hand are available at a fraction of the cost. Each PIN photodiode provides a separate output signal and if hundreds or thousands of PIN photodiodes are used with the outputs being processed in parallel, the large amount of processing circuitry required adds unacceptably to the overall cost of the detection device. It is desired to incorporate such low cost photodetector elements into a detection device, while also providing a reduced number of signals with a continuously varying relationship that indicate collective light position. One such an arrangement is disclosed and claimed in copending U.S. application Ser. No. 09/965,661, filed Sep. 27, 2001, and assigned to the assignee of the present invention. In the copending application, the disclosure of which is incorporated herein by reference, a plurality of photodetectors, such as PIN photodiodes, are arranged in a generally vertically oriented row. A weighting circuit is connected to each photodetector so that a portion of the output from an illuminated photodetector is provided as a first reference signal related in amplitude to the spacing of the illuminated photodetector from a first end of the row, and a portion of the output from the illuminated photodetector is provided as a second reference signal related in amplitude to the spacing of the illuminated photodetector from a second end of the row.
While this arrangement operates well, it is limited to relatively short photodetector arrays, since only a limited number of photodetectors can be connected to a tapped transformer weighting circuit to produce reference signals that define the illuminated photodetector position with sufficient accuracy. It is seen, therefore, that there is a need for a device and method for utilizing a plurality of photodetector arrays including a large number of photodetectors, and for accurately defining which one or ones of the photodetectors have been illuminated by processing a limited number of output signals that have a continuously varying relationship.